Measuring the amount of water in a subject's body is essential. Dehydration in a living body is a pathological condition in which water is reduced and often occurs when a person is exercising or when the temperature is high because a lot of water is excreted from the body by sweating to reduce an elevated body temperature.
Generally, when water, in a living body, is reduced by 3% or more of the body weight, body temperature regulation is disturbed. This disturbance of body temperature regulation causes the body temperature to increase, and then water in the living body is further reduced. In other words, a vicious cycle of dehydration is created, which leads to, at last, a pathological condition called heat illness. Heat illness includes conditions such as heat cramp, heat exhaustion, and heatstroke. In some cases, all the organs in the body are affected by heat illness.
Inpatients in hospitals and elderly persons in, for example, nursing homes tend to suffer from dehydration more seriously than healthy persons and non-elderly persons because most of them have lower capacities to regulate the body temperature. For that reason, for example, nurses in medical facilities such as hospitals and care persons in elder care facilities, such as, nursing homes need to regularly check and manage the amounts of body water in inpatients and elderly persons.